Topological measures of (70). Panel a: subset of the field lines in the region where the fields opposing twist units overlap. The field line helicity of the green field line indicated would have contributions due to its own complex geometry as well as its entanglement with the field. Panel b: field line helicity distribution (calculated using the code used in Prior & Yeates 2018) of (71) with t = 0, there is significant small scale structure indicating the field’s complex entanglement.